Phosphate and phosphors prepared therefrom

ABSTRACT

A new form of gamma - calcium pyrophosphate having a rhambohedral crystal shape and regular particle size, more especially in the range 8-8 1/2 microns, is prepared by controlled heating of anhydrous calcium hydrogen phosphate having a similar crystal form and particle size. The latter phosphate is prepared by precipitation of brushite and dehydration in an aqueous medium such as aqueous phosphoric acid at an elevated temperature. When the new pyrophosphate is employed as a source of calcium and phosphate ions in the preparation of luminescent phosphors, the products have enhanced luminescent brightness as compared with those produced from conventional calcium hydrogen phosphate.

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1451 Jan, 118, 1972 [54] PHQSPHATE AND PHUSPHORS PREPARED THEREFUM [72] Inventors: Joseph A. G. Bruce; Henry M. Levy; Pyliss Oborn, all of London, England 21 Appl. No.: 796,813

[56] References Cited UNITED STATES PATENTS 2,765,281 10/1956 Chrien "252/3014 P 3,505,012 4/1970 Dale et a1... 2,876,168 3/1959 Broge et a1. 2,901,400 8/1959 Thomas 3,012,852 12/1961 Nelson 3,095,269 6/1963 Chiola et al FOREIGN PATENTS 0R APPLICATIONS 927,353 5/1963 England ..252/301.4 P

OTHER PUBLICATXON S Rabatin et al., J of Physical Chemistry, pp. 491- 493, Vol. 64, Apr. 1960 Parodi et al., Electronic Paramagnetic Resonance Study of the Thermal Decomposition of Dibasic Calcium Orthophosphate, l 12 J. of the Electrochemical Society 688- 692 (No. 7,July 1965) Primary Examiner-0scar R. Vcrtiz Assistant ExaminerCharles B. Rodman Attorney-Owen J. Meegan [57] ABSTRACT A new form of y-calcium pyrophosphate having a rhambohedral crystal shape and regular particle size, more especially in the range 8-896 microns, is prepared by controlled heating of anhydrous calcium hydrogen phosphate having a similar crystal-form and particle size. The latter phosphate is prepared by precipitation of brushite and dehydration in an aqueous medium such as aqueous phosphoric acid at an elevated temperature. When the new pyrophosphate is employed as a source of calcium and phosphate ions in the preparation of luminescent phosphors, the products have enhanced luminescent brightness as compared with those produced from conventional calcium hydrogen phosphate.

1 Claims, 4 Drawing lFigu res PHOSPHATE AND PHOSIHOlRS PREPARED THEREFRUM This invention relates to the preparation of an improved form of gamma-calcium pyrophosphate, which can be used to advantage in the preparation of many well-known phosphors, such as pyro-, orthoand halo-phosphate phosphors, which find application in fluorescent discharge tubes and cathoderay tubes.

In the preparation of phosphors containing calcium and phosphate ions, the calcium and phosphate components are commonly supplied in the form of calcium hydrogen phosphate, which is prepared by conventional methods.

It has now been found that extremely pure crystalline 'ycalcium pyrophosphate can be prepared having regular particle size and rhombohedral crystal form. When this material is used as a component of a firing mixture in the preparation ofa phosphor, the resultant phosphor shows a higher lumen output than hitherto. For the brightest phosphors, it has been found that the 'y-pyrophosphate in accordance with this invention preferably has a particle size predominantly in the range of 8-8rmicrons.

In accordance with the invention, it has further been found that an improved y-calcium pyrophosphate, having such advantages in phosphor preparation, can be prepared by controlled heating of a form of anhydrous calcium hydrogen phosphate of regular particle size and crystal form, which can itself be made by a recrystallization technique under carefully controlled and critical conditions and which, on conversion to the 'y-pyrophosphate, retains its particular crystal shape and particle size distribution.

The preferred preparation of the y-pyrophosphate in accordance with the invention may be summarized as follows: pure precipitated brushite (Cal-IPO -2H is suspended in aqueous phosphoric acid at elevated temperature until it has been converted into anhydrous calcium hydrogen phosphate of the desired particle size and crystal structure; the anhydrous salt is separated off, washed and dried, and then converted to the desired 'y-pyrophosphate by heating to 500-600 C.

The reactivity of the 'y-phosphate formed is enhanced, so that the phosphors prepared from it are brighter than phosphors in the preparation of which the usual calcium hydrogen phosphate has been used.

The following more detailed description of the preparation of the required type of calcium hydrogen phosphate, and the conversion of the 'y-calcium pyrophosphate, given by way of example, will make the invention more readily apparent.

Purified aqueous diammonium hydrogen phosphate solution is slowly added to purified aqueous calcium chloride solution at a temperature of l8-20 C. until a pH of 6.7-7.0 is reached. It will be found that the required final pH is reached by using approximately 35 liters of calcium chloride solution, specific gravity l.l42, pH 7.0, and 50 liters of diammonium hydrogen phosphate solution, specific gravity 1.093, pH 7.2.

The brushite which is precipitated is filtered off, washed and suspended in 77.5 liters ofdeionized water which contains sufficient phosphoric acid to give a pH value of3.5.

This suspension is carefully stirred and the temperature raised at a rate not exceeding 3 C. per minute to 95 C. The temperature is maintained at 95 C. for 5 minutes. The brushite will then have been converted to anhydrous calcium hydrogen phosphate of the preferred particle size and crystal structure. The suspension is allowed to settle and the precipitate is removed by filtration. The precipitate is washed and carefully dried at l00120 C.

This method of preparation produces an especially pure, regular, anhydrous calcium phosphate with a particle size of 88/. microns as measured by the Fisher subsieve size method. Additionally, while the crystal unit cell is triclinic the crystal habit is approximately rhombohedral.

The conversion of this calcium hydrogen phosphate, to the required y-calcium pyrophosphate, is carried out by heating it in a suitable vessel to a temperature between 500600 C., for

example by heating in a silica crucible for 4 hours at 540 C. The crystal habit and particle size remain substantially the same. Care must be taken to complete the conversion, while at the same time avoiding the formation of the ,8- and otpyrophosphates.

The accompanying illustrations show the contrast between typical products of the known procedure and of the method according to the invention.

FIG. ll shows calcium hydrogen phosphate prepared by a conventional procedure, namely by precipitation at C. of aqueous calcium chloride, sp. gr. 1.1420 at 20 C., with sufficient aqueous diammonium phosphate, sp. gr. 1.1020 at 20 C. to give pH 3.5;

FIG. 2 shows 'y-calcium pyrophosphate prepared from the material of FIG. ll;

FIG. 3 shows calcium hydrogen phosphate prepared as described in the example above;

FIG. 4i shows 'y-calcium pyrophosphatc according to the invention, prepared from the phosphate of FIG. 3 as described in the example.

As an indication of the improved phosphor efficiencies obtainable by the use of the invention, 'y-calcium pyrophosphate, manufactured in the above way and use-d in the whole range of standard halophosphate phosphors, gave a consistent 5 percent increase in efficiency over phosphors made using the normal calcium hydrogen phosphate.

By way of more specific example, the following table shows lamp efficiencies obtained using typical phosphors manufactured from the 'y-calcium pyrophosphates according to the invention, compared with phosphors manufactured from the usual calcium hydrogen phosphate.

The phosphors were prepared by conventional techniques, by blending the following initial mixtures and firing in a crucifluorescent lamps of standard construction, and each value represents the average of six lamps.

0 hours hours Example 1. Using daylight-type halophosphate phosphor.

Phosphor with CaHPO, 3136 3063 Phosphor with 03,130, 3285 3115 Example 2. Using White-type halophosphate phosphor.

Phosphor with CaHPO, 3250 3176 Phosphor with Ca,P O, 34l0 3342 It will be appreciated by those skilled in the art of phosphor making, that all raw materials, firing procedures, postfiring treatments and suspension manufacture must be carefully controlled.

We claim:

l. A method of preparing gamma-calcium pyrophosphate phosphor comprising the steps of:

slowly adding purified aqueous diammonium hydrogen phosphate solution to purified aqueous calcium chloride solution at a temperature of l8-20 C. until a pH of 6.7-7.0 is reached and brushite is precipitated from the solution;

ly rhombohedral crystal habit;

removing the calcium hydrogen phosphate by filtration;

washing the calcium hydrogen phosphate;

drying the calcium hydrogen phosphate at a temperature of and heating the calcium hydrogen phosphate at a temperature between 500-600 C. to convert it to gamma-calcium pyrophosphate without substantial change in the particle size and crystal habit. 

